Systems and methods for synchronizing a plurality of RFID interrogators in a theatre of operation

ABSTRACT

RFID tags are used for many purpose including tracking. RFID interrogators are used to retrieve information from tags. In many applications, a plurality of RFID interrogators are required. Synchronization between interrogators in the same theatre of operation is critical to ensure that their broadcasts do not interfere with each other. In fixed RFID interrogator applications, RFID interrogators can be wired together allowing a channel to synchronize the transmissions of the RFID interrogators. Methods described herein can ensure that synchronization is maintained in the event of the failure of a synchronizing master. Furthermore, additional methods for synchronizing RFID interrogators in wireless applications are described allowing synchronization in the absence of wired connections between interrogators.

RELATED APPLICATIONS INFORMATION

This present application is a continuation of U.S. patent applicationSer. No. 14/225,359, filed on Mar. 25, 2014, entitled “Systems AndMethods For Synchronizing A Plurality Of RFID Interrogators in A TheatreOf Operation” which claims the benefit of priority as a Continuationunder 35 U.S.C. §120 of U.S. patent application Ser. No. 11/766,749,filed Jun. 21, 2007, entitled “Systems And Methods For Synchronizing APlurality Of RFID Interrogators In A Theatre Of Operation,” which inturn claims the benefit of priority under 35 U.S.C. 119(e) toProvisional Patent Application Ser. No. 60/805,423, entitled “RFID SmartCabinet And A Multidocument Read/Write Station,” filed Jun. 21, 2006,all of which are incorporated herein by reference as if set forth infull.

BACKGROUND

1. Field of the Invention

The field of the invention relates generally to Radio FrequencyIdentification (RFID) systems and more particularly to systems andmethods for synchronizing a plurality of RFID interrogators in a theatreof operation.

2. Background of the Invention

FIG. 1 illustrates a basic RFID system 100. A basic RFID system 100comprises three components: an antenna or coil 104, an interrogator 102with decoder 112, and a transponder, or RF tag 106 which is oftenelectronically programmed with unique information. Antenna 104 emitsradio signals 110 to activate and read and write data to tag 106.Antenna 104 is the conduit between tag 106 and interrogator 102, whichcontrols data acquisition and communication. Antennas 104 are availablein a variety of shapes and size, for example, in certain embodimentsthey can be built into a door frame to receive tag data from persons orthings passing through the door. In other embodiments, antennas 104 can,for example, be mounted on an interstate toll booth to monitor trafficpassing by on a freeway. Further, depending on the embodiments, theelectromagnetic field, i.e., radio signal 110, produced by an antenna104 can be constantly present when, e.g., multiple tags 106 are expectedcontinually. If constant interrogation is not required, then radiosignal 110 can, for example, be activated by a sensor device.

Often antenna 104 is packaged with interrogator 102. A conventionalinterrogator 102 can emit radio signals 110 in ranges of anywhere fromone inch to 100 feet or more, depending upon the power output and theradio frequency used. When an RFID tag 106 passes through anelectromagnetic zone associated with radio signal 106, it detects radiosignal 106, which can comprise an activation signal. In someembodiments, interrogators can comprise multiple antenna, thoughtypically only one transmits at a time.

RFID tags 106 come in a wide variety of shapes and sizes. Animaltracking tags, for example, inserted beneath the skin of an animal, canbe as small as a pencil lead in diameter and one-half inch in length.Tags 106 can be screw-shaped for insertion, e.g., in order to identifytrees or wooden items, or credit-card shaped for use in accessapplications. Anti-theft hard plastic tags that include RFID tags 106can be attached to merchandise in stores. Heavy-duty RFID tags can beused to track intermodal containers, heavy machinery, trucks, and/orrailroad cars for maintenance and/or tracking purposes. A multitude ofother uses and applications also exists, and many more will come intobeing in the future.

RFID tags 106 are categorized as either active or passive. Active RFIDtags 106 are powered by an internal battery at and are typicallyread/write, i.e., tag data can be rewritten and/or modified. An activetag's memory size varies according to application requirements. Forexample, some systems operate with up to 1 MB of memory. In a typicalread/write RFID work-in-process system, a tag 106 might give a machine aset of instructions, and the machine would then report its performanceto tag 106. This encoded data would then become part of the taggedpart's history. The battery-supplied power of an active tag 106generally gives it a longer read and write range. The trade off isgreater size, greater cost, and a limited operational life.

Passive RFID tags 106 operate without a separate external power sourceand obtain operating power generated from radio signal 110. Passive tags106 are consequently much lighter than active tags 106, less expensive,and offer a virtually unlimited operational lifetime. The trade off isthat they have shorter read ranges than active tags 106 and require ahigher-powered interrogator 102. Read-only tags are typically passiveand are programmed with a unique set of data, usually 32 to 128 bits,that cannot be modified. Read-only tags 106 often operate as a licenseplate into a database, in the same way as linear barcodes reference adatabase containing modifiable product-specific information. Not allpassive tags 106 are read-only tags.

RFID systems are also distinguishable by their frequency ranges.Low-frequency, e.g., 30 KHz to 500 KHz, systems have short readingranges and lower system costs. They are commonly used in securityaccess, asset tracking, and animal identification applications.High-frequency, e.g., 850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz, systemsoffer long read ranges, e.g., greater than 90 feet, high reading speeds,and are used for such applications as railroad car tracking andautomated toll collection, however, the higher performance ofhigh-frequency RFID systems 100 incurs higher system costs.

The significant advantage of all types of RFID systems 100 is thenoncontact, non-line-of-sight nature of the technology. Tags 106 can beread through a variety of substances such as snow, fog, ice, paint,crusted grime, and other visually and environmentally challengingconditions, where barcodes or other optically read technologies cannottypically be used. RFID tags 106 can also be read in challengingcircumstances at high speeds, often responding in less than 100milliseconds. RFID has become indispensable for a wide range ofautomated data collection and identification applications that would notbe possible otherwise.

A conventional RFID interrogator 102 comprises an RF transceiver 106 anda decoder 112. Decoder 112 can, for example, be a micro controller orother processing circuit configured to carryout the required functions.Often, decoder 112 is interfaced with memory 114. Firmware instructionsused by decoder 112 to control the operation of interrogator 102 can bestored in memory 114, along with RFID instructions that can becommunicated to RFID tag 106 and can be used to control acquisition ofinformation from RFID tags 106. Memory 114 can, depending on theembodiment, comprise one or more memory circuits.

FIG. 2 shows an example transmission operation of an RFID interrogator.Graph 200 shows a transmission of the RFID interrogator when no data istransmitted. At the start of each frame 202, interrogator 102 can beconfigured to transmit frame synchronization pulses 204, which can havea much shorter width than the period associated with frame 202. RFIDinterrogator 102 can transmit data to RFID tag 106 by modifying theframe synchronization pulses, for instance by doubling the pulses torepresent a binary “zero” and tripling the synchronization pulses torepresent a binary “one.” Graph 220 shows an example of such atransmission method by an RFID interrogator. Double pulses 222 and 230,which comprise two pulses sent within a short period compared to theframe period; represent the transmission of a “zero,” Triple pulse 226,which comprise three pulses sent within a short period compared to theframe period, represent the transmission of a “one.” Remaining singlepulses 224 and 228 do not represent data and synchronize the associatedframes.

Another method of modifying frame synchronization pulses used by RFIDinterrogators is to use wider pulses to represent a “zero” and stillwider pulses to represent a “one.” Graph 240 shows an example of such atransmission method. The “wider” pulses 242 and 250, which are stillshort compared to the frame period, represent the transmission of a“zero.” The “widest” pulse 246, which is still short compared to theframe period but wider than pulses 242 and 250, represent thetransmission of a “one.” The remaining “normal” width pulses 244 and 248do not represent data and synchronize the associated frames 202.

Graphs 220 and 240 illustrate just two possible examples ofcommunication protocols that can be used to facilitate transmission ofdata in system 100.

In response to interrogation signals from the interrogator 102, RFIDtags 106 can be configured to respond in the second half of frames 202Furthermore, in many embodiments of an RFID interrogation systems 100both tags 106 and interrogator 102 operate in the same frequency range.The synchronization pulses, whether “normal” or modified to carry datacan serve two additional purposes. First, the pulses can be used todefine the boundaries of frames 202 so the tags 106 can respond at theappropriate time. Second, the pulses supply power for passive RFID tags106.

FIG. 3 depicts an interrogation theatre 300 comprising a plurality ofinterrogators, of which interrogators 310 and 340 are shown forillustrative purposes. In addition, theatre 300 comprises a plurality oftags, of which tags 320, 322, and 344 are shown for illustrativepurposes. Tags 320, 322, and 344 can for example, be similar to, or thesame as, tag 106 described above. If allowed to operate independently,these readers can severely interfere with each other. To illustrate, inFIG. 3, RFID tags 320 and 322 are near interrogator 310, while RFID tag344 is near interrogator 340. Temporally, interrogator 310 has justtransmitted its request through its antenna 312 and is now awaiting aresponse signal from any nearby RFID tags. Because RFID tags 320 and 322are near to interrogator 310, they respond with RF signals 330 and 332,respectively; however, at approximately the same time, interrogator 340wishes to interrogate RFID tags nearby such as RFID tag 344, bytransmitting signal 346 through antenna 342. Since the responses 330 and332 are on the same frequency as the interrogation signal 346, andinterrogation signal 346 can be of greater power than signals 330 and332, interrogator 310 may only detect the signal from interrogator 340rather than from RFID tags 320 and 322.

FIG. 4 depicts the timing of the example given above. Graph 400 depictsinterrogator 310 attempting to interrogate nearby RFID tags using thecommunications protocol illustrated by graph 220. RFID tag 320 respondsand its RF output signal 330 is graphed over time in graph 410; however,with an unsynchronized RFID interrogator 340 also attempting tointerrogate nearby RFID tags as depicted in graph 420, associated signal346 can interfere with signal 330. As a result, antenna 312 sees thesignal depicted in graph 430, where rather than seeing pulses 412 and414 of signal 330 (graph 410), interrogator 310 is likely to seesomething like pulses 432 and 436 dominated by the influence of signal346 (graph 420) of interrogator 340. As a result, interrogator 310 mayinterpret pulses 434 and 438 of interrogator 340 as coming from RFID tag320, or interrogator 310 may just fail to code any signal or may receivecorrupted information.

SUMMARY

An RFID system comprises a plurality of synchronized RFID interrogators.Synchronization between interrogators in the same theatre of operationcan be critical to ensure that their broadcasts do not interfere witheach other. In fixed RFID interrogator applications, RFID interrogatorscan be wired together to allow synchronization of transmissions of theRFID interrogators.

In one aspect, synchronization is maintained in the event of the failureof a synchronizing master.

In another aspect, synchronizing RFID interrogators in the absence ofwired connections between interrogators is provided.

These and other features, aspects, and embodiments of the invention aredescribed below in the section entitled “Detailed Description.”

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and embodiments of the inventions are described inconjunction with the attached drawings, in which:

FIG. 1 is a diagram illustrating an exemplary RFID system 100;

FIG. 2 is a diagram illustrating example transmission protocols that canbe used in the system of FIG. 1;

FIG. 3 is a diagram illustrating an exemplary interrogation theatrecomprising a plurality of interrogators;

FIG. 4 is a diagram illustrating example signals and timing for thetheatre of FIG. 3;

FIG. 5 is a diagram illustrating an example baggage tracking system thatincludes a plurality of interrogators synchronized in accordance withone embodiment;

FIG. 6 is a diagram illustrating example signals and timing for thesystem of FIG. 5;

FIG. 7 is a diagram illustrating a temporal overview of a self-promotionprocess for synchronized interrogators in accordance with oneembodiment;

FIG. 8 is a flowchart illustrating an example method for interrogatorpromotion in accordance with one embodiment;

FIG. 9 is a flowchart illustrating an example method for adjusting framesynchronization pulses, when interference is detected in accordance withone embodiment;

FIG. 10 is a diagram illustrating an example embodiment of RFID tagresponse encoding in accordance with one embodiment; and

FIG. 11 is a diagram illustrating an example detecting interference inaccordance with one embodiment.

DETAILED DESCRIPTION

In one embodiment, synchronization signal 512 supplied bysynchronization master 510 can be used by interrogators 520, 530, and540 to ensure that the corresponding interrogator signals 524, 534, and544 do not interfere with reception of signals transmitted by RFID tags550-564. For example, if graphs 620, 630, and 640 correspond to signals544, 534, and 524, respectively, then it can be seen thatsynchronization signal 512 (graph 610) can cause each interrogator tobegin transmission at the start of a common frame period. In otherwords, interrogators 520, 530, and 540 can be configured such that eachinterrogator upon receipt of a pulse in signal 512. This can ensure thateach interrogator is finished transmitting before the start of thesecond half of frame 604, devoted by dashed line 606 during whichresponses from RFID tags 550-564 we received. Thus, interference signals524, 534, and 544 with those transmitted from RFID tags 550-564 can beavoided.

As mentioned above, the start 602 of frames 604, depending on therequirements of a particular implementation, begin some fixed period(Δd) after the rising edge of the pulses comprising signal 512 asillustrated on graph 610. The delay (Δd) can, for example, be longenough to account for various delays associated with the circuitrycomprising interrogators 520, 530, and 540.

In order to avoid the problem illustrated in FIGS. 3 and 4, for example,interrogators in a theatre of operation 300 can be synchronized asdescribed herein. FIGS. 5 and 6 illustrate an embodiment of a system 500with multiple interrogators in a single theatre of operation. In oneembodiment, for example, such a system can be employed in a baggagetracking system, e.g., at an airport.

FIG. 5 depicts a baggage tracking system 500 where a plurality ofinterrogators 520, 530 and 540 are synchronized in accordance with thesystems and methods described herein. In airport baggage tracking system500, the objective is to track the time and identity of each bag thatpasses by various checkpoints. To facilitate this objective, an RFIDinterrogator is placed at each checkpoint. Each bag is equipped with abaggage tag comprising an RFID tag. Upon the check-in, each bag isplaced on some sort of conveyance mechanism, such as a conveyor belt.RFID tags 550, 552, 554, 556, 558, 560, 562 and 564 represent the RFIDtags embedded in the baggage tags affixed on each bag. Each bagtraverses the checkpoint monitored by interrogator 540, then thecheckpoint monitored by interrogator 530, followed by the checkpointmonitored by interrogator 520.

Interrogators 520, 530, and 540 are coupled together and to asynchronization master 510, which is responsible for synchronizing theinterrogators. In this embodiment, the coupling is accomplished throughwiring 514. As depicted in FIG. 5, synchronization master 510 can be asimple pulse generator; however, in other embodiments one ofinterrogators 520, 530, and 540 can serve as a synchronization master.The master transmits, e.g., master 510 can be configured to transmit apulse train 512 to each of interrogators 520, 530 and 540. Interrogators520, 530, and 540 can be configured, upon receiving the synchronizationpulse, to transmit through antennas, 522, 532, and 542, respectively, aradio signal 524, 534, and 544, respectively, to interrogate passingRFID tags 550-564. Signals 524, 534, and 544 can be synchronizationpulses or can carry information, e.g., using the exemplary communicationprotocols illustrated in FIG. 2.

FIG. 6 illustrates an example of the signals and synchronization pulsestransmitted by interrogators 520, 530, and 540. Graph 610 depictssynchronization pulse train 512. Graph 620, 630, and 640 depict thesignal outputs of interrogators 520, 530, and 540, respectively. In someimplementations, the start 602 of the RF frames 604 do not correspondprecisely with the leading edges of the pulses in graph 610, becausethere can be some propagation delay in the circuitry associated withinterrogators 520, 530, and 540. A certain amount of inconsistency inthe delay can be tolerated, because responses to interrogation signalsare expected in the second half of the frame. As explained in detailbelow, each interrogator can transmit different signals withoutinterfering with other interrogator's ability to receive RFID tagresponses because regardless of the type of signal, all transmissions byall interrogators are concluded by the start of the second half of theframe 604 as illustrated by dashed lines 606.

Thus, RFID interrogators 520, 530, and 540 can be coupled to asynchronization master 510 configured to synchronize transmissions fromthe interrogators; however, in the event of a failure associated withsynchronization master 510, system 500 can lose its ability tosynchronize the signals of interrogators 520, 530, and 540. In oneembodiment, this is avoided by enabling one of the remaininginterrogators to become the synchronization master. Accordingly, whenemploying such a cooperative strategy, an interrogator can be in one oftwo states a synchronization master or a synchronization slave. Thus,one or more of the interrogators in a system configured to implementsuch a cooperative strategy must be able to both send and receive asynchronization signal.

FIG. 7 is a diagram illustrating a temporal view of signals generated ina system employing such a cooperative strategy. In this example, eachinterrogator in the system is capable of both sending and receiving aninterrogator signal. The system begins with an interrogator, oralternatively a signal generator, as a synchronization master configuredto generate a synchronization signal as described above and illustratedin graph 710. In this particular embodiment, there are three slaveinterrogators whose radio frame synchronization signals are depicted ingraphs 720, 730 and 740 and whose synchronization signals are depictedin graphs 725, 735, and 745. At 750, the synchronization master suffersa failure and ceases to generate the synchronization signal.

If just one of the interrogators in the system is capable of taking overas master, which is possible depending on the embodiment, then thatinterrogator will be promoted to master upon detecting the failure ofthe original synchronization master.

Such configurations can be sufficient to avoid synchronization failures;however, a potential drawback to such configurations is that there is nomechanism to ensure synchronization should one promoted interrogatorsubsequently fail, fails to generate of synchronization signal, or failsto be promoted. Thus, it can be preferable, depending on theimplementation, for a plurability of interrogators to be capable ofpromotion to master. In such embodiments, there must be some mechanismfor determining which interrogator will become the next master.

In one embodiment, the interrogators do not recognize an outage until apredetermined period of time has expired at 752. From there eachinterrogator selects a random period of time to wait before it attemptsto become the new synchronization master. Here, the first interrogatorselects the interval between 752 and 754. The second interrogatorselects the interval between 752 and 756, which happens to be a longerinterval. The third interrogator happens to randomly pick the intervalbetween 752 and 754, the same as the first interrogator. These waitintervals should be large compared to the frame period. In anotherembodiment, each interrogator at some point in its normal process canselect a random time out period before registering a failure of themaster. To use the same example, the period would be that between 750and 754 for the first interrogator, between 750 and 756 for the secondinterrogator, and between 750 and 754 for the third interrogator.

Each interrogator can be configured to send a pulse after the associatedwait period to the other interrogators indicating its attempt to becomethe master. The other interrogators, upon receiving the pulse, can beconfigured to remain slaves. The new synchronization master can thensend its synchronization signal to the other interrogators. A conflictcan arise in the unlikely event that two or more interrogators pulse atthe same time, which would be the case in the example above. In otherembodiments, various schemes can be used to avoid such conflicts, orcontentions. For example, in some embodiments, collision avoidanceschemes can be used. Factors such as skew in the docks of eachinterrogator can eventually lead to a dispersion of the pulsesgenerated. At this point, one of the interrogators will be seen aspulsing first relative to the others. This interrogator will then becomethe master and the others demoted to being slave interrogators. Forexample, at time 758, the pulse, generated by the third interrogatorbegins to trail those of the first interrogator. The third interrogatorcan be configured to detect that it is no longer the master, and ceaseto generate synchronization pulses at time 760.

FIG. 8 illustrates a flowchart illustrating an example method forinterrogator promotion in accordance with the systems and methods. Waitstates 810 and 850 represent the general waiting states for aninterrogator in the slave state and in the master states, respectively.For example, most interrogators start in wait state 810. They cantransition out of wait state 810 to step 812 if either a synchronizationpulse is received from another interrogator or a predetermined period oftime has elapsed since a synchronization pulse from a master wasexpected. This predetermined period is typically much larger than theframe period. If a synchronization pulse is detected, the interrogatorremains a slave and can perform its regular duties by sending either aframe synchronization pulse or data to an RFID tag at step 820, and ifappropriate, it can listen for RFID tag responses at step 822. Uponcompletion of the frame, the interrogator returns to wait state 810. Onthe other hand, if a synchronization pulse from a master has not beendetected, at step 814, a timeout interval is selected, e.g., randomlygenerated as described above, the timeout interval can be with apredetermined range, which is typically many times the frame period. Theinterrogator then waits at step 816 for either this timeout period toexpire or for a synchronization pulse from another interrogator.

If a synchronization pulse is received at step 818, the interrogatorremains a slave and can continue to perform its regular duties startingat step 820; however, if the timeout expires then the interrogatorattempts to become a master at step 854 by transmitting asynchronization pulse to all the other interrogators. It then cancontinue to perform its regular duties by sending either a framesynchronization pulse or data to an RFID tag at step 856 and then ifappropriate, it can listen for RFID tag responses at step 858. Uponcompletion of its regular duties, the interrogator returns to wait state850. In wait state 850, the interrogator waits for either the start ofthe next frame, which is one frame period after it sent the lastsynchronization pulse to the other interrogators, or for asynchronization pulse from another interrogator.

In step 852, if the interrogator detects a start of frame, it transmitsa synchronization pulse to the other interrogators in step 854 and theprocess repeats as before. But if the interrogator detects anothersynchronization from another interrogator, it ceases to be, a master,becomes a slave, and resumes slave duties at step 820. This can occur,for example, where the original master whose failure initiated thepromotion from slave to master of steps 814-854 comes back online. Thiscan also occur if during the promotion from slave to master one or moreother interrogators waited the same random interval and weresimultaneously promoted to master and over time, the internal clocks ofthe interrogators are skewed resulting in slight deviations in the pulseinterval.

Though extremely unlikely, there may be a situation where three or moreinterrogators claim to be masters. Thus, in certain embodiments, eachinterrogator can be configured to determine under such circumstancesthat one of the other interrogators is the rightful master, which willcause each interrogator to switch to a slave state. At this point, nosynchronization pulses are sent by any interrogator and the process foreach interrogator follows the diagram in FIG. 8 by traversing steps 812,814, 816 and 818. At which point, a new master is selected.Alternatively, skewing that results from differences in the tolerancesand errors associated with the circuitry of each interrogator can berelied on to eventually result in one interrogator being promoted overthe others as described above. Obviously, the more interrogatorsinvolved the longer such a process will take. Therefore, somealternatives as described above that reduces the delay involved can bepreferable for selecting among three or more contending masterinterrogators.

It should be noted that in another embodiment, a random predeterminedtimeout greater than the predetermined “master timeout” and less thanthe sum of the “master timeout” and the “random timeout” range could beused in wait state 810, thereby combining steps 812, 814, 816, and 818into a single branch point where the detection of a synchronizationpulse transitions the interrogator to step 820 and the expiration ofthis new predetermined timeout promotes the interrogator to a masterstate by transitioning to step 854. Such a hybrid timeout period can beused, for example, when an interrogator changes master-slave state, whena new frame is detected, when the tenth new frame is detected, etc.

Though the above embodiments address the synchronization issues relatingto the operation of multiple interrogators in a single theatre ofoperation, there are many applications where the wired approachdescribed in the preceding examples is not feasible, e.g., where theRFID interrogators are mobile such as those mounted on a forklift in awarehouse tracking application, or those used as hand-held scanners in ashipment tracking application. Accordingly, one or more wirelesscommunication links can be used to achieve synchronization. Any suchwireless approach should provide an inefficient use of power andspectrum associated with the wireless communication channel or link. Forinstance, ideally a master interrogator should be as centrally locatedas possible; however, in mobile applications, the interrogators can movearound in the theatre of operation. This can lead to inefficient use ofpower and spectrum since a master interrogator needs to generatesufficient power to be detected by even the most remote interrogator inthe theatre of operation. But since it is an objective to mitigateinterference between nearby interrogators, synchronization need only beenforced when interrogators are close enough to cause interference.Thus, in certain embodiments, interrogator synchronization is onlyemployed when interference from other interrogators is detected.

FIG. 9 is a flowchart illustrating an example method for adjusting framesynchronization pulses, when interference is detected, in accordancewith the systems and methods described herein. In step 910, theinterrogator waits for the start of frame. In step 912, the interrogatortransmits its frame synchronization or data to nearby RFID tags at thestart of the frame. The interrogator then waits, in step 914, for thesecond half of the frame. At step 916, the interrogator can attempt todetect any interference from other interrogators, while listening forRFID tag transmissions. If interference is detected at 918, theinterrogator delays, at step 920, its start of next frame time tocoincide with the start of frame it detected from another interrogatorat 918. If no interference is detected, the interrogator processes anyRFID tag transmissions it may have received at step 922. The processthen repeats.

Basically, if two interrogators come close enough to interfere, theinterrogator which is first to detect interference adjusts its framesynchronization timing to match the other interrogator. Because, theyare out of sync, one interrogator will have to be first in detectinginterference. The environment can become much more complicated if morethan two interrogators are out of frame synchronization, butrealistically that is unlikely, since the frame periods are typically onthe order of microseconds and physical movements take a much longertime, so by the time a third interrogator is out of sync with the firsttwo, those two should have synchronized.

There are many methods of distinguishing interrogation interference withRFID tag transmissions. Most of these methods involve incorporatingcertain patterns in the transmission protocol.

FIG. 10 illustrates a specific embodiment of such an encoding. Graph1010 shows an interrogator's frame synchronization pulses. An RFID tagcan transmit a “one” by sending a pulse in response in the second halfof a first frame and no pulse in a second frame as depicted in graph1030 and a “zero” by sending a no pulse in a first frame and a pulse inthe second half of a second frame as depicted in graph 1040. In theevent of no pulse as in graph 1020, there is no response from an RFIDtag. In the event of a pulse in the second half frame of both a firstand second frame, as in graph 1050, interference from another RFIDinterrogator can be deduced. More complex patterns in RFID tagstransmissions can be implemented, but often these complexities lead tomany more false readings.

FIG. 11 is a flowchart illustrating an example method forsynchronization that can alleviate some of the confusion associated withdetecting interference. In the example of FIG. 11, interrogators onlyattempt synchronization when not expecting RFID tag transmissions. In apractical system, RFID interrogators spend much of their time sendingframe synchronization pulses, but not expecting a return transmission.In step 1110, the interrogator waits for its internal clock to indicatea start of frame. In step 1112, the interrogator decides if it has apending transaction with an RFID tag, if so, at step 1114, it decideswhether it is waiting on a random count due to the detection ofinterference from a previous iteration. If so or if there is no pendingtransaction, the interrogator transmits a frame synchronization pulse atstep 1116. Otherwise, if there is a pending transaction and theinterrogator is not waiting a random count or that count has expiredhence no longer waiting, it transmits its data at step 1118. Theinterrogator then waits for the second half of the frame at 1120. Atstep 1122, the interrogator can attempt to detect any interference fromother interrogators, while listening for RFID to transmissions.

If interference is detected at 1122, the interrogator behavesdifferently depending on whether it is expecting data from an RFID tag.If it is not expecting data at step 1124, the interrogator delays, atstep 1126, its start of next frame time to coincide with the start offrame it detected from the interfering interrogator at 1122. If it isexpecting data, the interrogator selects a random number of frames towait in step 1128. If no interference is detected, the interrogatorprocesses any RFID tag transmissions it may have received at step 1130.The process then repeats. In the event multiple interrogators areattempting to interrogate at the same time, the random count gives aninterval when none of the interrogators are expecting data tosynchronize their respective frame synchronization pulses.

While certain embodiments of the inventions have been described above,it will be understood that the embodiments described are by way ofexample only. Accordingly, the inventions should not be limited based onthe described embodiments. Rather, the scope of the inventions describedherein should only be limited in light of the claims that follow whentaken in conjunction with the above description and accompanyingdrawings.

The invention claimed is:
 1. A radio frequency identification (RFID)interrogation system, comprising: multiple RFID interrogators, whereineach of the multiple RFID interrogators is configured to transmit aplurality of synchronization pulses to surrounding RFID tags, andwherein the multiple RFID interrogators includes a first RFIDinterrogator, the first RFID interrogator is configured to: transmit asynchronization to nearby RFID tags; while listening for RFID tagtransmissions in response to the synchronization pulse, detect anyinterference from another RFID interrogator among the multiple RFIDinterrogators; and if an interference is detected from another RFIDinterrogator among the multiple RFID interrogators, determine if data isexpected from a nearby RFID tag, and if no data is expected, synchronizea transmission of the next synchronization pulse with the nextsynchronization pulse of the another RFID interrogator; if interferenceis not detected from another RFID interrogator, wait for the expecteddata from the nearby RFID tag.
 2. The RFID interrogation system of claim1, the first RFID interrogator detects interference from another RFIDinterrogator when; a pulse is detected in the second half of a firstframe and a pulse is detected in a second half of a second frame; andthe first frame occurs immediately before the second frame.
 3. The RFIDinterrogation system of claim 1, wherein if no interference is detected,the first RFID interrogator is configured to process any RFID tagtransmission received during a second half of a frame.
 4. The RFIDinterrogation system of claim 1, wherein while synchronizing thetransmission of the next synchronization pulse, the first RFIDinterrogator is configured to delay the transmission of the nextsynchronization pulse so that the start of the next synchronizationpulse of the first RFID interrogator coincides with the start of thenext synchronization pulse of the another RFID interrogator.
 5. The RFIDinterrogation system of claim 1, wherein while waiting for the expecteddata from the nearby RFID tag, the first RFID interrogator is configuredto wait a random number of synchronization pulse periods.
 6. The RFIDinterrogation system of claim 1, wherein the first and the another RFIDinterrogators are wirelessly coupled to each other.
 7. The RFIDinterrogation system of claim 1, wherein the another RFID interrogatormoves into a wireless detection range of the first RFID interrogator. 8.The RFID interrogation system of claim 1, wherein at least one of thefirst RFID interrogator and the another RFID interrogator is a mobileRFID interrogator.
 9. The RFID interrogation system of claim 1, whereinthe first RFID interrogator is a mobile RFID interrogator mounted on aforklift for warehouse tracking.
 10. The RFID interrogation system ofclaim 1, wherein the first RFID interrogator is configured in form of ahand-held scanner for shipment tracking.
 11. A method for operating afirst radio frequency identification (RFID) interrogator among multipleRFID interrogators while avoiding interferences with other RFIDinterrogators among the multiple RFID interrogators, comprising:transmitting a synchronization pulse by the first RFID interrogator tosurrounding RFID tags; while listening for RFID tag transmissions inresponse to the synchronization pulse, detecting any interference fromanother RFID interrogator among the multiple RFID interrogators; if aninterference is detected from another RFID interrogator among themultiple RFID interrogators, determining if data is expected from anearby RFID tag, and if no data is expected, synchronizing atransmission of the next synchronization pulse of the first RFIDinterrogator with the next synchronization pulse of the another RFIDinterrogator; if interference is not detected from another RFIDinterrogator, waiting for the expected data from the nearby RFID tag.12. The method of claim 11, wherein detecting interference from anotherRFID interrogator includes: detecting a pulse in the second half of afirst frame, detected a pulse in the second half of a second frame; andthe first frame occurs immediately before the second frame.
 13. Themethod of claim 11, wherein if no interference is detected, the methodfurther comprises processing any RFID tag transmission received during asecond half of a frame.
 14. The method of claim 11, whereinsynchronizing the transmission of the next synchronization pulse of thefirst RFID interrogator with the next synchronization pulse of theanother RFID interrogator includes delaying the transmission of the nextsynchronization pulse of the first RFID interrogator so that the startof the next synchronization pulse of the first RFID interrogatorcoincides with the start of the next synchronization pulse of theanother RFID interrogator.
 15. The method of claim 11, wherein waitingfor the expected data from the nearby RFID tag includes waiting for arandom number of synchronization pulse periods.
 16. The method of claim11, wherein the first and the another RFID interrogators are wirelesslycoupled to each other.
 17. The method of claim 11, wherein the anotherRFID interrogator moves into a wireless detection range of the firstRFID interrogator.
 18. The method of claim 11, wherein at least one ofthe first RFID interrogator and the another RFID interrogator is amobile RFID interrogator.
 19. The method of claim 11, wherein the firstRFID interrogator is a mobile RFID interrogator mounted on a forkliftfor warehouse tracking.
 20. The method of claim 11, wherein the firstRFID interrogator is used as a hand-held scanner for shipment tracking.